In an internal combustion engine, a fuel-air mixture is burned so that the hot gaseous products exert a force on moving parts of the machine, doing useful work and generating power. In the internal combustion engine, the products of combustion act directly on pistons or turbine rotor blades. The first practical internal combustion engine was developed by Otto and Langen in 1876, which was the first four-stroke-cycle engine employing compression and operating on the principle of modern automobile engines.
The familiar four-stroke-cycle Otto engine employs a piston connected to a crank shaft, and disposed in a combustion cylinder connected to a fuel mixture source by an inlet valve and to an exhaust port by an exhaust valve. The piston first descends on the intake stroke, during which the inlet valve is held open. The fuel mixture is moved into the cylinder by the partial vacuum created by the descent of the piston. The piston then ascends on the compression stroke with both valves closed and the fuel mixture is ignited by an electric spark as the end of the stroke is approached. The power stroke follows, with both valves still closed and the gas pressure from the combustion of the fuel mixture into its gaseous raction products, acting on the piston because of its expansion. The exhaust stroke then completes the cycle with the ascending piston forcing the spent reaction products of the combustion pass the exhaust valve, now held open. The cycle then repeats itself, each cycle thus requiring four strokes of the piston; intake, compression, power and exhaust, and two revolutions of the crankshaft.
A wide variety of internal combustion engines have resulted from the requirements of an extensive range of possible applications. These various applications have been made in the automotive, aviation, marine and stationary fields. Of these, the automotive field is the most important because of the number of engines in use and the total power developed. However, all of these modern internal combustion engines employ petroleum products such as gasoline or fuel oil as their fuel. Indeed, the history of the internal combustion engine is contemporary with and dependent upon the development of the modern petroleum industry. Although petroleum has been known to man from ancient times, its use was limited to lighting, lubrication, and adhesive applications. The modern petroleum industry dates from the successful drilling of an oil well at Titusville, Pa. in 1859, which occurred just a few years before the development of the Otto four-stroke internal combustion engine.
By 1900, world production of petroleum had increased to 100 million barrels per year, stimulated by the increased uses found for the internal combustion engine. As the 20th century has progressed, fuels derived from petroleum have accounted for half of the world's total supply of energy and are used not only to power the engines of automobiles, tractors, trucks, ships, airplanes and missiles, but also the furnaces used to heat buildings. Either directly by combustion or indirectly through the generation of electricity, petroleum supplies most people of the world with artificial light. By the end of the 1970s, the world wide demand for petroleum has increased to over 60 million barrels per day. However, the world wide published, proven reserves of petroleum remain relatively fixed and, through the market manipulations by international cartels and through interruptions in the refining and distribution of petroleum products, critical shortages in the supply of petroleum products to consumers have begun to occur.
A great deal of attention has been recently brought to bear on alternate energy sources which can substitute for petroleum. Alternate fossil fuels such as coal are not in short supply and indeed it is estimated that at existing consumption levels, the proven reserves of coal could supply the world's needs for 400 years.
Interest in solar energy has also increased. Solar technologies can be classified into natural collection systems such as green plants and technological collection systems such as thermal or photovoltaic technologies. The simpler technology is the use of biomass for energy, which includes any form of matter that is living or was once part of a living organism, for example, leaves, wood, corn cobs, pea pods, algae, bacteria, kelp, and manure. Although the efficiency of conversion of solar energy by growing plants is only about one percent, the United States has 900 million acres of land under cultivation for lumber, paper and food. Either through direct burning or conversion to specialty fuels, the residues associated with forestry and agricultural operations alone could provide as much as 10 percent of the United States' energy needs. Densified biomass fuel is now being made from saw dust, bark, corn cobs, pea pods or coffee grounds by drying these materials to a moisture content of about 10% and compressing them into bricks or pellets. In this form, the biomass has an energy content higher than many coals and is easy to ship and store.
The basic practical problem now is how to convert existing solid fuels composed of coal or biomass into a suitable medium for combustion in an internal combustion engine or furnace.